There are four major synthetic resins used in the fabrication of most composite wood materials, for example plywood, particle board, wafer board, and glued wood construction products. The resins are used to bond the wood particles, wafers, or plies. The resins in order of volume used are: urea-formaldehyde; phenol-formaldehyde; melamine-formaldehyde; and resorcinol-formaldehyde. Current efforts are underway to produce wood composite adhesives that can be formulated without formaldehyde, which has been declared hazardous, even in the trace amounts found in wood composite products.
The phenol-formaldehyde or phenolic resins are typically polymerized, cross-linked resins synthesized with excess formaldehyde to ensure full polymerization. In use, they are typically sprayed or painted on the wood surfaces as a viscous liquid and cured under heat and pressure to produce a permanently solidified, cross-linked polymer.
There are two types of phenolic resins made; both types use as raw materials phenols and aldehydes, usually pure phenol and formaldehyde. Because phenol is produced primarily from petroleum, its price and availability are linked to that of petroleum. Consequently, phenolic resins are relatively expensive. In addition, the use of formaldehyde creates health and safety concerns. One type of phenolic resin, novolak, is made under acidic conditions; the acid catalyzes the reaction of phenol and formaldehyde to form the cross-linked polymeric resin. The other type of phenolic resin, resol, is made under basic conditions; a small amount of a base is added to the basic phenol to catalyze the reaction of phenol and formaldehyde into the cross-linked, polymeric resin. The catalyst may be precipitated from the product or remain in solution. The viscosity of the raw resin is typically increased to form the adhesive product by removing water and excess formaldehyde under vacuum distillation in order to keep the resin below its cure temperature. Resols are water-soluble resins which thermoset without additional reaction.
Because the price and availability of these synthetic phenolic resins is tied to that of petroleum, in the past decade a number of attempts have been made to produce from natural resources, phenol or phenolic analogs that could be substituted for the petroleum-based phenol in the synthesis of phenolic resins. Much of the work has centered on the use of the lignin waste of the paper pulping process and phenols extracted from biomass pyrolysis oils.
Most research into biomass pyrolysis, however, has been directed at producing charcoal or gas as fuels. In those processes, oil production is typically avoided by burning the gas at a temperature above the oil dew point. In the biomass pyrolysis work in which the pyrolysis oil has been collected as a product, the processes have typically been geared toward maximizing oil production by pyrolyzing the biomass at a relatively low temperature of less than approximately 500.degree. C. in entrained flow and fixed-bed reactors. The resulting oil typically includes an abundance of phenolic ethers; at that temperature alkyl phenolic production is low. In one series of tests, pine sawdust was pyrolyzed at approximately 500.degree. C. in a vortex reactor. The pyrolysis oil was then extracted with ethyl acetate to produce a phenolic-rich fraction comprising only about 25% of the starting oil. That fraction was then used as a phenol substitute in resol production. However, the low yield and expensive separation techniques make that process commercially impractical.